Description: Define restricted description binder. In case it doesn't exist, we return
a set which is not a member of the domain of discourse A . See also
comments for df-iota . (Contributed by NM, 15-Sep-2011)(Revised by Mario Carneiro, 15-Oct-2016) WARNING: THIS "AXIOM", WHICH IS THE OLD
df-riota , CONFLICTS WITH (THE NEW) df-riota AND MAKES THE SYSTEM IN
set.mm INCONSISTENT. IT IS TEMPORARY AND WILL BE DELETED AFTER ALL USES
ARE ELIMINATED.